A lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). The lithographic projection apparatus can include a mask having a circuit pattern corresponding to an individual layer of the IC. This pattern can be imaged onto a target portion (e.g., comprising one or more dies) on a substrate (e.g., a silicon wafer) which has been coated with a layer of radiation sensitive material (e.g., resist). The radiation sensitive material can be developed, and the substrate further processed, in order to form the circuit pattern on the substrate. In general, a single wafer will contain a plurality of target portions that are successively irradiated via a projection system of the lithographic projection apparatus. For example, in a wafer-stepper lithographic projection apparatus, the entire mask pattern is imaged onto the target portion at the same time. In contrast, in a step-and-scan lithographic projection apparatus, the mask pattern is imaged onto the target portion by scanning the mask pattern in a given direction (i.e., the scanning direction) while, e.g., synchronously scanning the substrate anti-parallel to the scanning direction. Since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate is scanned will be a factor M times the speed that the mask is scanned.
In a manufacturing process using a lithographic projection apparatus, the substrate may undergo various processes in order to form a single layer of the IC, such as priming, resist coating, and soft baking. Furthermore, after imaging the mask pattern onto the layer of radiation sensitive material on the substrate, the substrate may be subjected to additional processes, e.g., post-exposure bake (PEB), development, hard bake, and measurement/inspection of the imaged pattern. The resulting patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., in order to complete the layer. If multiple layers are required, this process, or a variant thereof, can be repeated for each additional layer. Accordingly, an array of devices can be formed on the substrate. These devices can then be separated from one another by dicing or sawing, and packaged individually.
The lithographic projection apparatus includes a projection system, which hereinafter may be referred to as the “lens.” However, this term should be broadly interpreted as encompassing various types of projection systems including, e.g., refractive optics, reflective optics, and catadioptric systems. The lithographic projection apparatus further includes a radiation system configured to direct, shape, or control a beam of radiation. The beam of radiation can be patterned with, e.g., a mask or reticle, and projected onto the substrate. The lithographic apparatus can have two or more substrate tables (and/or two or more mask tables). In such “multiple stage” lithographic apparatus, the additional substrate tables can be used in parallel, or preparatory steps can performed while one or more other substrate tables are being used for exposures. For example, U.S. Pat. No. 5,969,441, herein incorporated by reference, describes a twin stage lithographic apparatus.
The masks or reticles comprise geometric patterns which correspond to the circuit components formed on the substrate. Mask and reticle patterns are generated using CAD (computer-aided design) programs, this process often being referred to as EDA (electronic design automation). Most CAD programs implement design rules determined by processing and design limitations in order to create functional masks and reticles. For example, design rules may define the space tolerance between circuit devices (such as gates, capacitors, etc.) or interconnect lines, so as to ensure that the circuit devices or lines do not conflict. The design rule limitations are typically referred to as “critical dimensions” (CD). A critical dimension of a circuit can be defined as the smallest width of a line or hole, or the smallest space between two lines or two holes. Thus, the CD determines the overall size and density of the circuit.
One objective of IC fabrication is to accurately reproduce the circuit design on the substrate using the mask. However, as the critical dimensions of the target patterns decreases, it is more difficult to reproduce the target pattern on the substrate. Double exposure is a multiple exposure technique which allows the minimum CD capable of being reproduced on the substrate to be reduced. For example, using dipole illumination, the vertical edges (i.e., features) of the target pattern are illuminated in a first exposure, and the horizontal edges of the target pattern are illuminated in a second exposure.
Another double exposure technique separates the features of the target pattern into two or more different masks, and each mask is imaged separately to form the desired pattern. This technique may be used when the features of the target pattern are spaced too closely for the features to be imaged. Accordingly, the target pattern may be separated onto two or more masks such that the features on a given mask are spaced sufficiently far apart that each feature can be imaged. As a result, it is possible to image target patterns having features spaced too close together to be imaged using a single mask by ensuring that the pitch between the features on a given mask is greater than the resolution limits of the projection system. Indeed, this double exposure techniques allows for k1<0.25. Nevertheless, limitations exist with conventional double exposure techniques. For example, conventional methods for decomposing target patterns operate on each feature of the target pattern as a unit, rather than on smaller portions of the features. As a result, it is not possible to obtain a k1<0.25 for certain target patterns, despite the use of double exposure. Additionally, conventional decomposition methods are often rule based algorithms which require too many rules to implement complex designs. Moreover, rule based algorithms may fail when situations or conflicts arise for which no rule has been defined. Accordingly, it is desired to overcome the deficiencies of conventional multiple exposure techniques and methods for decomposition of a target pattern onto multiple masks.